This invention relates to a hydrogen-oxygen solid electrolyte fuel cell characterized by using Cu.sub.2 O as an oxidizing agent electrode (cathode) catalyst.
The fuel cell is a power generating apparatus for producing electric current by electrochemical reaction of a fuel with an oxidizing agent. Owing to its high power generating efficiency, the fuel cell has come to attract increasing attention. Today, research and development on the fuel cell are well advanced.
The hydrogen-oxygen solid electrolyte fuel cell is basically constructed of a fuel electrode (anode), an oxidizing agent electrode (cathode), a solid electrolyte intimately interposed between the two electrodes, and housings for supplying hydrogen and oxygen (or air) respectively to the fuel electrode and the oxidizing agent electrode. The following reactions take place at these electrodes:
______________________________________ Fuel electrode H.sub.2 .fwdarw. 2H.sup.+ + 2e.sup.- Oxidizing agent electrode 1/2 O.sub.2 + 2e.sup.- .fwdarw. O.sup.2- " O.sup.2- + 2H.sup.+ .fwdarw. H.sub.2 O Overall reaction H.sub.2 + 1/2 O.sub.2 .fwdarw. H.sub.2 O ______________________________________
The electromotive force (potential difference) which is produced by the aforementioned reaction is theoretically 1.23 V at 25.degree. C. Actually, it is on the order of about 0.8 to 0.9 V because the aforementioned reaction is accompanied by secondary reactions such as the formation of H.sub.2 O.sub.2 when an acidic electrolyte is used, for example. The polarization inside the cell which functions as resistance to the generation of electric current occurs in varying forms, including the resistance polarization which is the resistance offered by the electrolyte itself, the activation polarization (ionization polarization and reaction polarization) which takes place at the electrodes, and the concentration polarization which arises during supply of the reaction gas. Owing to these various polarizations, the cell voltage declines in proportion as the electric current increases.
For the purpose of obtaining as high cell voltage and as high electric current as possible, therefore, it is imperative that the aforementioned secondary reactions and polarizations should be repressed to the fullest possible extent. To this end, the development of an electrolyte possessing high proton conductivity and the development of an electrode catalyst, particularly an oxidizing agent electrode catalyst, are required among all the catalysts of this kind, platinum type catalysts now stand out in various respects. Because of practical difficulties such as high cost and scarcity of supply, however, there persists a demand for a new catalyst which can take the place of such platinum type catalysts. Fuel cells of the type adapted to operate at high temperatures involve reactions of high rate and do not especially require catalysts. In the case of solid electrolyte fuel cells adapted to operate at low temperatures such as room temperature as contemplated by the present invention, development of a new electrode catalyst proves to be an indispensable requirement.
The inventors have already developed solid electrolytes which possess high proton conductivity (namely, low resistance polarization), i.e. 12-molybdophosphoric acid, H.sub.3 Mo.sub.12 PO.sub.40.29H.sub.2 O (hereinafter referred to as "12-MPA") and 12-tungstophosphoric acid, H.sub.3 W.sub.12 PO.sub.40.29H.sub.2 O (hereinafter referred to as "12-WPA") exhibiting conductivity of 0.2 mho.multidot.cm.sup.-1 at 25.degree. C. (O. Nakamura et al., Chem. Lett., 1979, 17-18).
For the development of a hydrogen-oxygen solid electrolyte fuel cell of the type which can operate at low temperatures, however, the development of an oxidizing agent electrode catalyst capable of taking the place of a platinum type catalyst as described above is an essential requirement similarly to the development of a solid electrolyte of high proton conductivity.
Such an oxidizing agent electrode catalyst as suits the purpose just mentioned is disclosed by U.S. Pat. No. 3,899,357, for example. This is a composite of oxides of transition metals including copper. It is, however, too expensive to be economically feasible. It has much room for further improvement.
An object of this invention is to provide a novel oxidizing agent electrode catalyst which far excels the aforementioned known catalyst in terms of feasibility and takes the place of a platinum type catalyst advantageously.